Minor optimization of SVG <textPath> layout

Path::pointAtLength and Path::normalAngleAtLength both call
Path::pointAndNormalAtLength to do the work.
SVGTextLayoutEngine::layoutTextOnLineOrPath call the two former, which
means it'll setup and process the path twice.

Change the two calls into one call to pointAndNormalAtLength to reduce
the work required.

BUG=135310

Review URL: https://codereview.chromium.org/135853005

git-svn-id: svn://svn.chromium.org/blink/trunk@165078 bbb929c8-8fbe-4397-9dbb-9b2b20218538